The present invention generally relates to power amplifiers and, more particularly, to a digitally controllable predistortion system and method for compensating for nonlinearities within the power amplifier
A typical power amplifier, such as a traveling wave tube amplifier (TWTA), exhibits a nonlinear characteristic 100 as illustrated in FIG. 1. A linear region is defined as the range up to the point where increasing the RF input signal results in gain compression. The input power versus output power characteristic shown, also known as, amplitude to amplitude modulation (AM/AM) curve, rolls off from the linear region as the input power increases and the output power reaches saturation. This effectively means that the amplifier gain decreases in the nonlinear region while the efficiency of the amplifier is at a maximum.
In addition gain compression effects, typical power amplifiers also exhibit nonlinear amplitude to phase modulation (AM/PM) effects (not shown). When using higher order modulation schemes with multi-amplitude symbols, additional unintentional phase modulation occurs when operating in the nonlinear region.
Typically, the power amplifier is operated at an operating point 120 which is xe2x80x9cbacked offxe2x80x9d from its maximum output capacity in order to maintain linearity. Even though the power amplifier is operating in the xe2x80x9cbacked offxe2x80x9d linear region, the amplifier still consumes power and has a low input power to output power conversion efficiency.
A known method to compensate for amplifier nonlinearities includes predistortion techniques. A nonlinear pre-equalizer is tuned for a particular amplifier and predistorts an input signal to the amplifier to compensate for the distortion that occurs to the signal after going through a TWTA with certain AM/AM characteristics. The predistorted input signal typically experiences a desired gain expansion with input power so that the combined effect is linear gain up to saturation. By this method an extrapolated transfer characteristic 110 is approached.
This approach suffers from the disadvantage of being amplifier specific, that is, each amplifier requires a specifically tuned nonlinear pre-equalizer. As is well known in the art, this is costly in terms of both hardware and manufacturing time expended in tuning each nonlinear pre-equalizer. More particularly, in satellite applications, payloads are increased by the requirement that each TWTA have it""s own nonlinear pre-equalizer.
A known predistortion system is disclosed in U.S. Pat. No. 6,342,810 to Wright et al. The system includes a data structure in which each element stores a set of compensation parameters including finite impulse response (FIR) filter coefficients for predistorting a wideband input transmission signal. The parameter sets are preferably indexed within the data structure according to multiple signal characteristics, such as instantaneous amplitude and integrated signal envelope, each of which corresponds to a respective dimension of the data structure. To predistort the input transmission signal, an addressing circuit digitally generates a set of data structure indices from the input transmission signal, and the indexed set of compensation parameters is loaded into a compensation circuit which digitally predistorts the input transmission signal. The process of loading new compensation parameters into the compensation circuit is preferably repeated every sample instant, so that the predistortion function varies from sample to sample. The sets of compensation parameters are generated periodically and written to the data structure by an adaptive processing component that performs a non-real time analysis of amplifier input and output signals. The system includes a digital compensation signal processor, a generalized digital to analog converter, an RF upconversion block coupled to a nonlinear amplifier, an amplifier sampling structure, an RF downconversion block, a generalized analog to digital converter, and an adaptive control processing and compensation estimator.
As can be seen, there is a need for a digitally controllable system and method for compensating for nonlinearities within a power amplifier. The described system is adaptable to a plurality of power amplifiers. Further, the described system is easy to implement, and reduces implementation cost of the system by eliminating the need for a separate linearizer for each traveling wave tube amplifier.
In one aspect of the present invention, a digitally controllable nonlinear pre-equalizer circuit for receiving an input signal and generating an output signal includes a splitter for dividing the input signal into a first signal path and a second signal path; an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal; a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the mixer and vector modulator operable to generate a nonlinear signal; and a summer for summing the linear signal and the nonlinear signal to generate the output signal.
In another aspect of the present invention, a digitally controllable nonlinear pre-equalizer circuit for receiving an input signal and generating an output signal includes a splitter for dividing the input signal into a first signal path and a second signal path; an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal; a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the processor chip being operable to store a plurality of power amplifier parameters corresponding to a plurality of power amplifiers, each power amplifier parameter being provideable as the digital signal, the mixer and vector modulator operable to generate a nonlinear signal; and a summer for summing the linear signal and the nonlinear signal to generate the output signal.
In yet another aspect of the present invention, an amplifier system for receiving an input signal and generating an output signal includes a power amplifier; a splitter for dividing the input signal into a first signal path and a second signal path; an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal; a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the mixer and vector modulator operable to generate a nonlinear signal; and a summer for summing the linear signal and the nonlinear signal to generate a gain expansion signal, the gain expansion signal being input to the power amplifier, the power amplifier providing the output signal.
In a further aspect of the present invention, a method of digitally controlling a nonlinear pre-equalizer circuit receiving an input signal and generating an output signal includes dividing the input signal into a first signal path and a second signal path; providing an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal; providing a digital to analog converter coupled to a processor chip; providing a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from the digital to analog converter, the processor chip providing a digital signal to the digital to analog converter, the mixer and vector modulator operable to generate a nonlinear signal; and summing the linear signal and the nonlinear signal to generate the output signal.
In yet another aspect of the present invention, a satellite system for receiving an input signal and generating an output signal includes a satellite; a power amplifier disposed on said satellite; a nonlinear pre-equalizer circuit coupled to said power amplifier, said nonlinear pre-equalizer circuit including a splitter for dividing the input signal into a first signal path and a second signal path; an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal; a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the mixer and vector modulator operable to generate a nonlinear signal; and a summer for summing the linear signal and the nonlinear signal to generate a gain expansion signal, the gain expansion signal being input to the power amplifier, the power amplifier providing the output signal.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.